1. Technical Field
The present invention relates generally to Ball Grid Array (hereinafter "VGA") packages. More particularly, the present invention relates to a method of forming interconnections having mixed solder joint profiles within BGA packages to increase fatigue life of the BGA interconnections.
2. Related Art
In the manufacture of BGA packages, differences in the coefficients of thermal expansion between a chip carrier or module and a board creates stresses, in particular, shear stress, within the interconnections, or solder joints. The stresses are typically the highest in the solder joints at the corners of the BGA package, and in the solder joints directly beneath the corners and edges of the chip. Frequently, the solder joints in these regions cannot withstand the stresses applied over many on/off cycles, resulting in fatigue failure of the BGA solder joints. It is well known that elongating the solder joints will extend the fatigue life. It was determined that elongated solder joints are more compliant and have lower shear stress than when compared to shorter solder joints having the same volume.
Several techniques have been used in the industry to produce elongated solder joints. For instance, spacers, high-melt solder columns, and other additional materials, have been placed between the module and the board to force the solder joints to elongate. Lifting forces have been applied to the BGA packages during solidification to extend the solder joints. Solder joints having increased volume have been placed at selected locations within the package thereby forcing the other solder joints to elongate, and so on.
However, some of these techniques are incompatible with the trend toward reducing the size of semiconductor packages. Other techniques entail a complicated assembly process which increases manufacturing costs and reduces production yields. Further, some techniques decrease the space on the substrate available for wiring.
Accordingly, there exists a need in the industry for a BGA package exhibiting an increased fatigue life, without increasing the pad size, solder volume, size of the board, etc., or raising the costs.